Ehm. It's public. Anyone can verify this by looking at the transaction volume in the various currencies, multiply and add.
It's actually not that simple, as traders with higher 30-day volumes are given discounts on the trading fees. Even a relatively small trader could get down to rates of 0.50% and larger traders, who probably make up a significant percentage of the volume, could get as low as 0.30% without too much trouble.
https://www.mtgox.com/fee-scheduleI would guess that their revenue is more in the $6-7 million range, and is potentially lower than that, depending on what percentage of the total trade volume is accounted for by the largest traders.